has_many_distinct_on.rb

# BAD each iteration loads invoice model

class Company < ApplicationRecord
  has_many :invoices
end

class Invoice < ApplicationRecord
  belongs_to :company
end

Company.active.each do |company|
  SMSService.msg(company.contact_number, company.invoices.last&.total_cost)
end


# GOOD eager load latest invoices

class Company < ApplicationRecord
  has_many :invoices
  has_one :latest_invoice, -> { merge(Invoice.latest) }, class_name: :Invoice, inverse_of: :company  
end

class Invoice < ApplicationRecord
  belongs_to :company

  scope :latest, -> do
    from(
      <<~SQL
        (
          select distinct on (company_id) invoices.*
          from invoices
          order by company_id, id desc   
        ) invoices
      SQL
    )
  end    
end

Company.active.includes(:latest_invoice).each do |company|
  SMSService.msg(company.contact_number, company.latest_invoice&.total_cost)
end